Method and apparatus for the production of high tenacity polyolefin sheet

ABSTRACT

A process for the production of virtually full density polyolefin suitable for further processing by drawing to form a high tenacity, highly oriented polyolefin sheet comprising: a) feeding a metered amount of polyolefin powder into the nip between two heated calender rolls; b) rolling the powder through the nip under these conditions until a coherent sheet of polyolefin is produced. According to a highly preferred embodiment, initially, the nip is set at a gap smaller than the size of the smallest polyolefin powder particle and at a temperature above the melting point of the powder and once a coherent sheet of polyolefin exits the nip the temperature in the nip is lowered to a temperature below the melting point of the polyolefin powder and the gap increased to a desired level above the thickness of the largest powder particle.

This application is a continuation of U.S. patent application Ser. No.12/287,799 filed Oct. 14, 2008 now U.S. Pat. No. 7,858,004, which is acontinuation-in-part of U.S. patent application Ser. No. 12/080,197filed Apr. 1, 2008 now U.S. Pat. No. 7,854,870.

FIELD OF THE INVENTION

The present invention relates to ultra high molecular weightpolyethylene (UHMWPE) and other high molecular weight polyolefinmaterials useful for ballistic applications and more particularly to anovel and highly economical process for their production.

BACKGROUND OF THE INVENTION

The processing of ultra high molecular weight polyethylene (UHMWPE),i.e. polyethylene having a molecular weight in excess of about 2million, is known in the polymer arts to be extremely difficult.Products made from such materials are, however, very strong, tough anddurable.

In the following series of U.S. patents filed by Kobayashi et al andassigned to Nippon Oil Co., Ltd. a number of inventions related to thefabrication of fibers and films of polyolefins generally and UHMWPEspecifically, are described: U.S. Pat. Nos. 4,996,011, 5,002,714,5,091,133, 5,106,555, 5,200,129, and 5,578,373. The processes describedin these patents generally describe the continuous production of highstrength and high modulus polyolefin films by feeding polyolefin powderbetween a combination of endless belts disposed in an up and downopposing relationship, compression molding the polyolefin powder at atemperature below its melting point between the endless belts and thenrolling and stretching the resultant compression molded polyolefin intoan oriented film. As compression molded, the sheet is relatively friablethus requiring the subsequent calendering or drawing operations toprovide an oriented film that exhibits very good strength and durabilityproperties. In fact, the strength of such materials produced by theseprocesses can be 3 times that of steel on a weight basis and theyexhibit very low creep.

Enhanced processes for the production of such materials have also beendescribed in the following U.S. patents and patent applications: U.S.Pat. No. 7,348,053 and U.S. patent application Ser. No. 11/217,279 filedSep. 1, 2005.

A common element of all of these prior art processes is that theyrequire compaction of an UHMWPE powder as the initial step in theproduction process. Until now, it has been the thinking of the UHMWPEmanufacturing community that such powder compaction was necessary inorder to place the material in a form that it could be subsequentlyrolled and drawn as described in the referenced prior art. Stateddifferently, it has been the thinking that in order to produce theproduct in a process involving the subsequent rolling and drawing stepsto obtain the orientation required for the production of ballisticallyuseful UHMWPE, the powder had to first be placed in the form of a sheetthat demonstrated sufficient tenacity to be successfully processed insuch subsequent rolling and drawing processes. In the prior art, such aform was obtained by compacting the powder into a relatively friablesheet that could be introduced into the rolling operation for subsequentprocessing.

The performance of this compaction process step, particularly in theproduction of UHMWPE sheets wider than 1-2 inches in width, requires theuse of relatively massive, quite complex and very expensive equipment(measured in the millions of dollars for installed such equipment). Suchequipment thus requires high levels of capital expenditures forinstallation and due to its complexity ongoing high operating andmaintenance expenses. Additionally, compaction, as practiced in theprior art requires that the polymer be evenly distributed across anddownweb (in the machine direction). Small variations in distribution(unequal mass) create defects in the sheet when it is subsequentlycalendered. Calendering requires that the mass entering the nip besubstantially equal across the gap opening. If a streak or longitudinalarea of the compacted sheet is low in polymer mass; it creates lowdensity material in that area when it exits the calender. This resultsin a weak area in the film that can break during subsequent drawing, orleaves a weak place in the finished product. By introducing the polymerat the calender, the material can redistribute to a small degree and theresulting sheet has a much more consistent density. Even and consistentpolymer distribution is one of the largest issues with any process thatincludes compaction.

U.S. Pat. No. 4,436,682 to Knopp, issued Mar. 13, 1984 describes aprocess for compacting polymer powders into fully dense products.According to this patent, a polymer powder is fed from a hopper into thenip between two rolls, compacted therein at a temperature below themelting point of the polymer powder and withdrawn from the nip undertension to form a “fully dense” polymer sheet. According to Knopp, whenhis process is applied to an UHMWPE powder, the resulting sheet has adensity of about 0.82 g/cc which he designates as “substantially fullydense”. It is well known that the density of UHMWPE is on the order ofabove 0.945 g/cc. Hence, the product of Knopp's process is hardly “fullydense” and is unsuited to further processing by calendering or drawing,since it will tear or break when subjected to such processes.Additionally, the temperature requirements recited by Knopp are suchthat subjecting, for example UHMWPE, to such temperatures during rollingto form the coherent sheet would destroy the properties of the UHMWPEmaking it unsuitable for subsequent calendering or for the applicationof the produced sheet to ballistic or other high impact applications.

It would thus be of great benefit to the producer of such UHMWPEmaterials, particularly in widths greater than a couple of inches, if amuch simpler, smaller and less expensive first process step could besubstituted for the powder compaction step, without negatively affectingthe either the product thus produced or significantly affecting thekinetics of the process, i.e. it did not, for example, slow productionto an uneconomical rate.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide anenhanced process for the production of UHMWPE sheet that eliminates theneed for the previously described compaction step and uses a much morecost effective and simpler process for the production of a high tenacityUHMWPE sheet that can undergo subsequent processing by drawing.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process for theproduction of virtually full density polyolefin suitable for furtherprocessing by drawing to form a high tenacity, highly orientedpolyolefin sheet comprising: a) feeding a metered amount of polyolefinpowder into the nip between two heated calender rolls; b) rolling thepowder through the nip under these conditions until a coherent sheet ofpolyolefin is produced. According to a highly preferred embodiment,initially, the nip is set at a gap smaller than the size of the smallestpolyolefin powder particle and at a temperature above the melting pointof the powder and once a coherent sheet of polyolefin exits the nip thetemperature in the nip is lowered to a temperature below the meltingpoint of the polyolefin powder and the gap increased to a desired levelabove the thickness of the largest powder particle. Such a process notonly eliminates the need for a separate and costly compaction step, butyields a coherent polyolefin sheet that is ready for drawing inaccordance with prior art processes for the production of a hightenacity, highly oriented polyolefin sheet having a high heat of fusion.According to a further highly preferred embodiment of the presentinvention, the polyolefin of choice is ultra high molecular weightpolyethylene (UHMWPE).

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the production processes of theprior art.

FIG. 2 is a schematic diagram of a preferred embodiment of the apparatusused to implement the drawing portion of the preferred process of thepresent invention.

FIG. 3 is a schematic side view of the apparatus used to producecoherent UHMWPE sheet in accordance with the present invention.

FIG. 4 is a schematic side view of an alternative apparatus used toproduce coherent UHMWPE sheet in accordance with an alternativeembodiment of the present invention.

DETAILED DESCRIPTION

In the description that follows, operating parameters, materialproperties etc. are presented in the context of those for ultra highmolecular weight polyethylene (UHMWPE), but it will be readilyunderstood by the skilled artisan in the polymer field that theinvention described herein is readily applicable to other polyolefinpolymers such as high molecular weight polypropylene through thejudicious selection of materials and process conditions appropriate forthese other polyolefin materials.

The term “tape” or “ribbon” as used herein refers to products havingwidths on the order of or greater than about ½ inch and preferablygreater than 1 inch. The term “fiber” as used herein is meant to definea “narrow” tape, i.e. an element narrower than about ½ inch. The term“slit film fiber” refers specifically to a “fiber” or narrow tape madein accordance with the present invention that exhibits a generallyrectangular cross-section and smooth, i.e. non-serrated or ragged edges.The terms “sheet” and “film” as used herein are meant to refer to thinsections of the materials of the present invention in widths up to andexceeding 160 inches in width as could be produced in large commercialequipment specifically designed for production in such widths. Accordingto a preferred embodiment, such sheets, films, and tapes have agenerally rectangular cross-section and smooth edges. Hence, thefundamental difference between a “tape”, a “ribbon”, a “slit filmfiber”, a “fiber”, a “film” and a “sheet” as used to describe theproducts of the processes described herein relates to the width thereofand is generally independent of the thickness thereof. The term “fullydense” as used herein in connection with the production of a coherentUHMWPE sheet, as defined hereinafter, is meant to define a coherentUHMWPE sheet that exhibits a density above about 0.94 g/cc.

Referring now to FIG. 1, the processes described in the prior art anddepicted schematically in FIG. 1 comprised the continuous production ofhigh strength and high modulus polyolefin films by feeding polyolefinpowder between a combination of endless belts disposed in an up and downopposing relationship, compacting the polyolefin powder at a temperaturebelow its melting point between the endless belts and then rolling andstretching the resultant compression molded polyolefin into an orientedfilm. To the extent of their relevance to the modified processesdescribed herein, the aforementioned prior art descriptions contained inU.S. Pat. Nos. 4,996,011, 5,002,714, 5,091,133, 5,106,555, 5,200,129,and 5,578,373. are incorporated herein by reference in their entirety.

A major difference between the processes of the prior art and those ofthe present invention is that the present invention obviates the needfor the compaction step and its related high cost entirely while alsoimproving the reliability of the process through the even distributionof polymer powder across and along the formed polymer sheet. Thus, oneof the methods described herein begins with heated polyolefin powderintroduced as described hereinafter directly into a pair of heated,counter rotating calender rolls under very specific temperature and gapconditions to produce a coherent polyolefin sheet suitable forsubsequent further drawing to orient the polyolefin and to produce aballistically or high impact condition useful high tenacity, highlyoriented polymeric material.

According to one preferred embodiment of the present invention, thepolyolefin processed in accordance with the process of the presentinvention is an UHMWPE that exhibits high crystallinity (above about 75%as determined by differential scanning calorimetry), a heat of fusionequal to or greater than 220 joules/gram and low levels of entanglement.Thus, it is preferred that the input starting material UHMWPE possessthe degree of crystallinity and heat of fusion and meet the lowentanglement requirements stated above. Such commercially availablematerials as Ticona X-168 from Ticona Engineering Polymers, 2600 UpdikeRoad, Auburn Hills Mich. 48236 and type 1900 CM from Basell Corp. 2801Centerville Road, Wilmington, Del. 19808 are useful in the successfulpractice of the present invention.

Referring now to the accompanying drawings, as depicted in FIG. 3, theinitial step in the process of the present invention utilizes a directroll apparatus 10 comprising a polymer powder hopper 12 that feeds ametered amount of polymer powder 14 into a vibratory chute 16 via ametering device 18 and thence to a containment plate 20. At containmentplate 20 the powder is introduced into the gap or nip 22 between twocounter rotating heated calender rolls 24 and 24A rotating in thedirections shown by arrows 26 and 26A. A heater 27 preferably aninfrared heater, imparts heat to powder 14 as described more fullybelow. Heater 27 is preferably located from about 2 to about 8 inchesabove powder 14 in vibratory chute 16 and set at a temperature ofbetween about 160 and 220° F. These distances and temperatures will, ofcourse, be variable depending upon the particular polymer powder 14being processed, and the type of heater used, but have been foundsuitable for the processing of the preferred UHMWPE. As powder 14cascades down vibratory chute 16 onto containment plate 20 it builds toa point where it is drawn into gap 22.

The successful practice of this embodiment of the present inventionrequires that at the start of the direct roll process, gap 22 be setnarrower than the size of the smallest individual polymer powderparticle, for example at about 50μ. Gap 22 may, of course, be widened ifthe minimum particle size of polymer powder 14 is greater than 50μ.Similarly, at start up of the direct rolling process described herein,heated calender rolls 24 and 24A are heated to a temperature above themelting point of polymer powder 14. While this melting point will bedependent upon the particular material being processed, in the case ofthe preferred UHMWPE starting materials described elsewhere herein thisinitial temperature can be as high as about 149° C. or about 3° C. abovethe melting point of the preferred UHMWPE. Lower temperatures could, ofcourse, be appropriate for lower melting polyolefin materials. At thispoint, rolling of powder 14 is initiated. As soon as a coherent sheet ofpolymer 28 begins to emerge from gap 22 the temperature of calenderrolls 24 and 24A is reduced to below the melting point of polymer powder14 and gap 22 is increased to that desired for the final productthickness for coherent sheet 28. As used herein, the term “coherentsheet” is meant to define a polymer sheet that is suitable for furtherprocessing by drawing without tearing, ripping or otherwise becomingunusable in such additional processing. For all practical purposes, sucha sheet will be virtually fully dense such as in the case of thepreferred UHMWPE materials described herein having a density above about0.945 g/cc. For the preferred UHMWPE powders 14 described elsewhereherein the operating temperature (the temperature after formation of acoherent sheet 28 is in the range of from about 136 to about 144° C. andpreferably between about 139 and about 141° C., and the operating gap ison the order of 100μ and 230μ and preferably at about 140μ. It should benoted that the initial and operating temperatures recited herein are notnecessarily set points for the polymer powder/sheet in nip 22, butrather surface temperatures of heated calender rolls 24 and 24A.

While the operating speed of the apparatus just described will vary withthe particular polyolefin being processed, using the preferred UHMWPEmaterials described above, start up roll speeds of between about 1.9 toabout 4.0 meters per minute have been found acceptable. Steady stateoperation of the apparatus is generally within the range of betweenabout 2.0 and about 12.0 meters per minute. It should be noted thatthese operating speeds are based primarily on one's ability to take upcoherent sheet 28 and the size of heated calendar rolls 24 and 24A,since larger rolls will generally tend to increase the surface incontact with the polymer in nip 22. Thus, if downstream operations ortake up apparatus are capable of faster speeds, or larger diameter rollsare used, higher operating speeds for the direct roll process justdescribed are possible.

The product of the just described process is a virtually full dense andtranslucent UHMWPE sheet, i.e. an UHMWPE sheet having a density of about0.95 to about 0.98 g/cc.

While the apparatus used to practice the process of the presentinvention is depicted herein as vertically oriented, the process willoperate equally well in a horizontal configuration, i.e. with thepolymer powder being fed to gap 22 between two horizontally parallelcalender rolls 24 and 24A. In this alternative orientation, powder 14 ismetered from a heated hopper located above horizontally parallelcalender rolls 24 and 24A so that powder 14 is fed from above into gap22 and the product sheet 28 is drawn from below gap 22. All otheroperating procedures, i.e. temperature control and gap settingvariations remain the same.

While not critical to the successful practice of the present invention,and clearly variable depending upon the particular polyolefin beingprocessed, roll surface roughnesses of from about 4 to about 8 RMS havebeen found suitable for the processing of the preferred UHMWPE materialsdescribed herein.

Referring now to accompanying FIG. 4 that depicts a schematic side viewof the alternative apparatus 70 useful in the successful practice of thealternative process of the present invention, rolls 24 and 24A accordingto this embodiment are oriented horizontally as opposed to vertically aspreviously described in connection with FIG. 2. As used in thedescription that follows, the following terms indicated by the referencenumerals shown in FIG. 4 have the following meanings and purposes:doctor blade gap 74 controls the amount of polymer laid on rolls 24 and24A and finally introduced into the nip 76. For the horizontal rollarrangement shown in FIG. 4, the mass (amount) of polymer laid on therolls must equal the mass (amount) that exits nip 76 as a sheet; (if themass is higher, it will build up above nip 76 and eventually spill overthe side as waste or create such a high nip pressure the rolls gap willbe forced open and/or the roll torque required to turn the rolls willbreak a mechanical component or stall the drive motor); roll nip or rollgap 76 is the closest distance between opposing rolls 24 and 24A (thisis the main parameter that controls rolled sheet thickness); roll nipreservoir 78 is the small reservoir of polymer that sits just above rollnip 76 (in a horizontal arrangement, the polymer touches both rolls andis brought into the nip by friction and compression from both rolls,while in the vertical arrangement previously described (see FIG. 3), thematerial is slightly self regulating since it can rest mainly on roll24A and fall back out of nip 76 if too much powder is present).

In accordance with the embodiment depicted in FIG. 4, polymer 14 issupplied from a hopper 80 that includes a hopper gate 82 that helps tocontrol the flow of polymer 14 from hopper 80. Since, as describedbelow, heating of polymer 14 takes place as polymer 14 contacts and isrotated on periphery 86 of roll 24, it is preferred that hopper 80 notbe heated, although some small amount of heat can be imparted to polymer14 while it is resident in hopper 80 and such imparting of minimalamounts of heat, i.e. at temperatures significantly below the meltingpoint of polymer 14 (for example below the 160° F. to 220° F. preheattemperatures used in the previously described “vertical” process) shouldbe considered as within the meaning of the term “unheated” as used todescribe hopper 80 in the appended claims. A doctor blade 84 regulatesthe flow of polymer 14 from hopper 80 onto roll 24 wherefrom it istransported about the periphery 86 of roll 24 into roll nip reservoir78. It is preferred, but not absolutely necessary, that doctor blade 84be vibrated. The setting of doctor blade 84 is determined by sensors(not shown) that detect the amount of powder in the roll nip reservoir78, the presence and amount of powder, if any, at the horizontalextremes of roll nip reservoir 78 and the force between the rolls. Thedesign, fabrication and operation of such devices are well within theskills of the skilled artisan and, accordingly, are not described indetail herein. Coherent sheet, tape, ribbon, film or fiber 28 emergesfrom roll nip 76 as in the case of the “vertical” process describedabove.

In order to better understand the operation of the apparatus justdescribed, it is important to understand the role and the operation ofdoctor blade 84. The doctor blade gap 74 controls the amount of polymer14 delivered to the roll nip 76. Roll nip gap 76 controls the amount ofpolymer 14 removed from roll nip reservoir 78. Roll nip reservoir 78acts not only as a filter to maintain this balance, but also serves as ameans of forcing material into roll nip gap 76. If polymer 14 isdelivered from doctor blade gap 74 at a higher rate than it is removed,roll nip reservoir 78 will overflow or become so large that the pressurein roll nip 76 will open roll nip 76, break a mechanical component, orstall the motor. If polymer 14 exits roll nip reservoir 78 faster thanit is fed in, the rolled sheet will have insufficient polymer and havevoids.

Doctor blade gap 74 and the roll speed control the quantity of polymer14 delivered to nip 76. The laydown on roll 24A does not need to be asuniform as required by the prior art double belt process of Kobayashidue to the presence of roll nip reservoir 78. Thus, roll nip reservoir78 serves as a small quantity reservoir (peaks and valleys in thelaydown are accumulated and redistributed) and it also allows polymer 14to move sideways to a small extent.

Similarly, it is important to fully understand the function of roll nipreservoir 78. For a given roll gap 76, the product thickness will remainwithin a fairly narrow range. The height of roll nip reservoir 78 aboveroll nip 76 determines how much of polymer 14 is in contact with rolls24 and 24A and the diameter of rolls 24 and 24A also play a function. Asrolls 24 and 24A rotate, polymer 14 is brought closer to roll nip 76 andthe density of polymer 14 is increased as it approaches roll nip 76until at some point it reaches the maximum density for polymer 14. Ifthe volume of polymer 14 in roll nip reservoir 78 is higher, polymer 14reaches its maximum density earlier in the rotation of the rolls asshown by arrows 26 and 26A. If the amount of polymer 14 in roll nipreservoir 78 is too small, polymer 14 is brought into roll nip 76 andcompressed into a compacted sheet but as it approaches roll nip 76, thesheet never reaches full density before it is finally rolled into asheet at or near the closest point between the rolls, in the center ofroll nip 76. In other words, at a given doctor blade gap, a certainamount of polymer 14 is laid on roll 24. The roll speed determines howfast this material is rotated around and fed into the roll nip (gap) 76.However, at this point, it becomes a balancing issue. It is desired thatan exit rolled sheet have a given thickness and strength. If polymer 14is delivered into roll gap 76 in an insufficient amount, and roll gap 76is held constant, roll nip reservoir 78 will partially empty and thesheet density will fall. The lower density will initially show up as aloss in strength, followed by voids of sufficient size to make theproduct more opaque. Since insufficient pressure will be exerted on thepolymer particles they will not bind together as well and as thisbinding strength and compression falls, the polymer 14 strength willfall along with increasing voids.

While not absolutely necessary to the successful practice of the presentinvention, it is desirable to use side or end dams (not shown in theaccompanying drawing) to limit lateral movement of polymer 14 againstdoctor blade 84.

While the size (diameter), finish and speed of rolls 24 and 24A can varybroadly, certain parameters for these elements have been found to beparticularly useful in the successful practice of this embodiment of thepresent invention. Rolls of 9 and 12 inches in diameter have been foundparticularly useful, but rolls of larger and smaller diameters can beused providing certain minimum pressures are maintained in roll gap 76.With larger rolls increased speeds are possible due to the longercontact time between the roll and polymer 14 as it rotates in contactwith periphery 86. For rolls about 12 inches in diameter, speeds of fromabout 1 up to about 12 m/min are possible while rotation speeds ofbetween about 1 and to about 3 m/min have been found satisfactory withroll rotation speeds of about 2.3 meters per minute producing highlydesirable results with the particular UHMWPE materials being processed.Speed conditions for other roll diameters processing similar UHMWPEpolymers are generally as follows: for a 15″ diameter roll (1.2 meterscircumference), speeds of about 2.88 m/min are highly desirable; and fora 24″ roll the optimum speed approaches 4.6 m/min.

Roll surfaces must be hard and possess good wear and abrasioncharacteristics. The surfaces should be consistently smooth to provideconsistent movement of the polymer into roll nip 76. Chrome surfaces inthe 1-15 RMS range are preferred. Markings and discoloration of thesurfaces have an impact on process performance. If the roll surfaceshows discoloration, more or less heat transfer occurs. This can leave aminor mark on the product and at worse a weak spot.

Again, while roll gap 76 can vary widely depending upon the polymerbeing processed, the diameter of rolls 24 and 24A, etc. for theproduction of UHMWPE sheet in a thickness range of from about 0.05 andabout 0.25 mm in accordance with this embodiment, roll gaps of betweenabout 0.015 and about 0.35 mm have proven satisfactory with rolls of asize as specified herein. These ranges should be considered merely asguidelines as wider or narrower roll gaps may be necessary or desireddepending upon the various parameters, especially roll diameter,described hereinabove.

As with the previously described vertical roll process, certain start-upparameters are desirable, but, depending upon the polymer material beingprocessed, not absolutely necessary. According to one preferredembodiment, it is preferred that at start up the rolls be heated to atemperature of from about 1 to about 5 degrees above the melting pointof the polymer being processed and the roll gap be set at from about 20to about 50 microns smaller than the operating roll gaps suggestedabove. Under operating conditions, for the processing of UHMWPE, theroll temperatures should be in the range of from about 130° C. and about150° C., preferably in the range of from about 132° C. and about 143°C., and most preferably between about 136° C. and about 140° C. Whilethese start up conditions are preferred for certain of the UHMWPEmaterials discussed herein, they are not critical and in many instancesthe gap and temperature requirements at start up are similar to thoseused during operation. For example, in certain cases, operatingtemperatures of between about 130° C. and 146° C. will be satisfactorywith narrower temperature ranges of from about 136.5° C. to about 142°C. desirable at roll speeds of between about 1 and about 4 meters perminute. Roll gaps at start up will generally be about the same as thoseused under operating conditions, i.e. between about 0.015 and about 0.35mm using the materials and conditions described herein. Applicants donot, however, wish to be bound to specific narrow start up and operatingconditions as these will vary widely depending upon the material beingprocessed, the desired product sheet thickness, the roll temperature,the roll speed, the roll size, the roll finish, etc.

As will be apparent to those skilled in the art, it would be an obviousalteration of the instantly described “horizontal” process to use a pairof mirror imaged hopper 80, and doctor blade 84 arrangements to obtain acoherent sheet 28. Such an alteration of apparatus 70 can result inhigher operating speeds.

One very significant advantage of the just described alternative processis that it broadens somewhat the range of useful startingmaterials/polymers. Using this “horizontal” process just described,UHMWPE polymers having molecular weights as low as 1 million andexhibiting heats of fusion as low as about 190 joules per gram andconcomitantly lower degrees of crystallization have been successfullyprocessed into ballistic and high impact resistant sheets, ribbons,tapes, films and fibers that exhibit properties similar to those ofmaterial produced in accordance with the “vertical” roll processdescribed above and the modified Kobayashi process described in U.S.Pat. No. 7,348,053.

Post-processing of coherent sheet 28 to obtain a highly useful UHMWPEballistic or impact resistant sheet, film, tape, ribbon or fiber isperformed in much the same fashion as and in apparatus similar to thatdescribed in issued U.S. Pat. No. 7,348,053, issued Mar. 25, 2008, i.e.by drawing coherent sheet 28 which are referred to and incorporatedherein in their entirety.

Referring now to FIG. 2, the drawing apparatus 40 utilized to achievethe thickness reduction and strength increase of the coherent sheetproduced as just described that result in production of the preferredUHMWPE products of the present invention 10 comprises:

a payoff 42, a godet stand 44 including heated godet rolls 46 (to annealthe product) and nip rolls 48 for establishing and maintaining tensionin the line, a first draw zone 50, a first in-line tension sensor 52, asecond godet stand 54, a second draw zone 56, a second in-line tensionsensor 58, and unheated take-up rolls 68. As seen from FIG. 1, the inputor starting material of this process is generally the thick, compactedand rolled but unoriented product of the compaction step of the priorart production process. According to the preferred process of thepresent invention, the input or starting material in thedrawing/calendaring process steps described below is, of course,coherent sheet 28 that emerges from gap 22 in the process describedabove.

Each of the elements of the apparatus just described and utilized in thesuccessful practice of the present invention are well known in the filmand fiber drawing arts as is their combination in a line of the typejust described. Consequently, no detailed description of such a line isrequired or will be made herein and the reader is referred to thenumerous design manuals and descriptions of such apparatus commonlyavailable in the art.

Maintaining a constant tension of between about 0.5 and about 5g/denier, and preferably between about 0.8 and 3 g/denier during drawingis also important to the production of product having the required“thinness” and other enhanced properties specified herein. The term“denier” as used herein is defined as the weight in grams of 9000 metersof the product film, tape, sheet or fiber. At tension levels below 0.5g/denier reductions will be obtained, but the enhanced propertiesdiscussed above may not be fully achieved, while at tension levels aboveabout 5 g/denier the material will tend to separate. In the case ofdrawing, tension is a function of the feed polymer and can vary broadlydepending thereon and the ranges just specified refer to those founduseful with particular preferred UHMWPE commercial starting materials.

According to a highly preferred embodiment of the present invention,drawing is performed in line with direct rolling as describedhereinabove. In such a continuous process, calender rolls 24 and 24Abecome payoff 42 of drawing apparatus 40. Such an arrangement provides ahighly efficient method for practicing the novel production process ofthe present invention.

After thickness reduction by drawing in the apparatus shown in FIG. 2according to the processing parameters just described, the UHMWPE films,sheets, fibers or tapes thus produced exhibit heats of fusion at orabove about 243 joules/gram, tenacities in the range of from about 18and 20 g/d, tensile moduli between about 1200 and about 1800 g/d andelongations in the range of from about 1.6 to about 2.0 percent.

There have thus been described novel processes for the production ofcoherent polyolefin, preferably UHMWPE, sheet and high tenacity, highlyoriented polyolefin, preferably UHMWPE, sheet, film, tape, ribbon orfiber that eliminates the need for the prior art compaction step which,until the development described herein, was considered necessary for thesuccessful production of such materials.

As the invention has been described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

What is claimed is:
 1. A process for the production of a coherentvirtually fully dense polyolefin sheet suitable for further processingby drawing to produce a high tenacity, highly oriented film, tape,fiber, ribbon or sheet from a polyolefin powder comprising: a) providinga pair of horizontally aligned counter rotating calender rolls separatedby a nip and a metering device in alignment with a first of saidcalender rolls; b) feeding a metered amount of polyolefin powder intosaid nip between said counter rotating heated calender rolls; c)controlling the amount of polyolefin powder at said nip by adjustingsaid metering device; and d) rolling the powder through the nip underthese conditions until a coherent fully dense polyolefin sheet isproduced.
 2. The process of claim 1 wherein the polyolefin powdercomprises an ultra high molecular weight polyethylene powder.
 3. Theprocess of claim 2 wherein said ultra high molecular weight polyethylenepowder exhibits high crystallinity, a molecular weight above 1 million,a heat of fusion equal to or greater than 190 joules/gram, and lowlevels of entanglement.
 4. The process of claim 1 wherein the coherentfully dense sheet has a density above 0.945 g/cc.
 5. The process ofclaim 1 wherein said nip includes an operating gap; said operating gapis between 0.015 mm and 0.35 mm; and said counter rotating calenderrolls include surface temperatures between 136° C. and 144° C.
 6. Theprocess of claim 1 wherein said counter rotating heated calender rollsare rotate at a rate of between 1 and 12 meters/minute.
 7. The processof claim 1 wherein said drawing is performed in-line and continuouslywith the formation of said coherent fully dense sheet.
 8. A process forthe production of high tenacity, highly oriented polyolefin sheet havinga high heat of fusion comprising: a) providing a pair of horizontallyaligned counter rotating calender rolls separated by a nip and ametering device in alignment with a first of said calender rolls; b)feeding a metered amount of polyolefin powder into said nip between saidcounter rotating heated calender rolls; c) controlling the amount ofpolyolefin powder at said nip by adjusting said metering device; d)rolling the powder through the nip until a coherent fully dense sheet ofpolyolefin is produced: and e) drawing the coherent fully dense sheet ofpolyolefin under controlled tension to produce a high tenacity, highlyoriented polyolefin sheet having a high heat of fusion.
 9. The processof claim 8 wherein said polyolefin powder comprises an ultra highmolecular weight polyethylene powder.
 10. The process of claim 9 whereinthe ultra high molecular weight polyethylene powder exhibits highcrystallinity, a molecular weight above 1 million, a heat of fusionequal to or greater than 190 joules/gram and low levels of entanglement.11. The process of claim 8 wherein the fully dense coherent sheet has adensity above 0.945 g/cc.
 12. The process of claim 8 wherein said nipincludes an operating gap; said operating gap is between 0.015 mm and0.35 mm; and said counter rotating calender rolls include surfacetemperatures between 136° C. and 144° C.
 13. The process of claim 8wherein the counter rotating calender rolls rotate at a rate of between1 and 12 meters/minute.
 14. The process of claim 8 wherein said drawingis performed in-line and continuously with the formation of saidcoherent fully dense sheet.